Homogeneous rabbit antibodies to streptococcal carbohydrates, are used to investigate structural aspects of the allotypic and idiotypic markers of immunoglobulins. The partial amino acid sequence was determined for a rabbit light chain bearing latent b9 allotype. In addition, structural work on a rabbit heavy chain bearing the a3 allotype is presently under way. Studies such as these that combine serological and structural investigations of antibodies have the potential to define genetic mechanisms involved in antibody synthesis.